Peter was having a hard time understanding the person and work of Jesus Christ. Peter got the person of Jesus right. Jesus is the Christ the Son of the living God. Peter did not understand the work of Jesus. When Jesus was describing what His work on earth entailed, Peter basically said, "No way, Jesus, that will never happen to You!" Peter had his mind on earthly things. He thought that since Jesus really is THE Christ that He should have worldly power and glory - that He should rule mightily here on earth.
What Peter did not understand is that the labor of the cross is where true glory is found. By living the perfect life that we are unable to, dying the death our sin deserves, and rising victoriously from the grave - Jesus has overcome sin, death, and the devil and given us the promise of eternal life! That is true glory! As Jesus Himself said in Matthew 20:28, "The Son of Man came not to be served but to serve, and to give His life as a ransom for many."
We too, like Peter, tend to have our minds focused on earthly things. There’s always a subtle temptation to believe that, because we’re Christians, life should go easier than it does. You’ve suffered some sort of setback that has you reeling and saying, "I thought God was more powerful than this. I thought He was more faithful than this. Why has Jesus failed me?"
The truth is that we’ve got the person of Christ right, but like Peter we’ve got the labor of Christ wrong. We want miracles and wonders and power on demand, not mercy and strength and patience in tribulation. We want Jesus to labor with might and power, but Jesus still ministers to us chiefly with mercy and grace. We want immediate deliverance from affliction: yet Jesus, who used His suffering on the cross to deliver us from sin, death, the power of the devil and the gates of hell, still labors through suffering—including the crosses you bear—for your good. To accuse God of being faithless during trial is simply not true, for the One who has already shown His faithfulness by dying on the Cross cannot be anything but faithful to you now.
The law here boils down to this: do not base your understanding of God’s mercy and faithfulness on how life is going. Hear of His mercy and faithfulness in His Word, and rejoice in it despite how life may be going. Because this is true: Christ took up His Cross that you might be His follower. He laid down His life to gain life for you. Risen again, He has given you new life in your baptism, and He continues to grant you grace by His Word and Supper. As His way of saving the world perplexed Peter, so His way of permitting suffering will perplex you; but just as the Lord did not abandon
Peter, neither will He abandon you. The Day is coming when the Son of Man is going to come with His angels in the glory of His Father, and many rightly fear that Judgment Day. You do not. Rather, you know that you are already innocent before God for the sake of Jesus, and you know that Judgment Day is the day when your sufferings are finally ended and only life and glory remain. The labor of the Cross is finished. Forgiveness, life, and salvation are yours through the finished work of Jesus Christ!